There are many applications in which it is desirable to provide an indication of when the combustion air present at a flame is inadequate for satisfactory combustion, but still sufficient to maintain the flame. Typically, some of the combustion air for the flame is supplied as part of a primary air-gas mixture, and the remainder is supplied as secondary air. If the amount of combustion air supplied to the flame is gradually reduced below a predetermined minimum level adequate to produce satisfactory combustion, the flame will at first continue to persist, but combustion will be incomplete, and it is only after the supply of combustion air has been reduced substantially farther that the flame will actually be extinguished. Consequently, if one merely uses a simple conventional flame sensor to turn off the gas supply when the flame extinguishes, this will not prevent the flame from continuing to burn when the combustion air level is below the minimum adequate level for satisfactory combustion but above the flame-extinction level. Permitting combustion to continue with inadequate levels of combustion air not only wastes fuel, but is also pollutive of the atmosphere and/or may produce undue quantities of potentially harmful combustion gases such as carbon monoxide.
One application of the invention, with reference to which it will be particularly described, is in connection with the main gas burner for a domestic hot-air furnace using a heat exchanger, in which a primary air-gas mixture is supplied under pressure to the interior of the main burner body and exits at the main burner ports, the flame at the burner ports also being supplied with secondary air which typically flows first along the bottom of the main burner, then upward along the sides of the burner to the flame area; the combustion products heat the interior of the heat exchanger, and are then vented through an appropriate flue, which flue is an extension of the passage provided for the flow of secondary air. Such flow of secondary air and combustion products is typically by natural thermal convection.
It has been found that if there is a perforation in the wall of the heat exchanger which separates the combustion products from the chamber through which the room air to be heated is circulated, or if there is a substantial blockage in the flue, the normal flow of secondary air to the vicinity of the flame may be substantially reduced to below the minimum adequate level for satisfactory combustion, even though the flame persists, with the above-mentioned drawbacks of fuel inefficiency, environmental pollution and possible danger. Since the flame does not become extinguished under these assumed circumstances, it is not possible to detect the undesired reduction in secondary air by merely detecting absence of the flame.
Devices are known in the prior art which can, to some extent at least, detect the quality and extent of combustion in a flame, for example certain types of heat and radiation sensors which have been used on large industrial furnaces. However, such devices are typically quite complex and costly, and in fact may in some instances be more costly than an entire domestic hot air furnace.
Also known are combustion-sensitive pilot-flame devices, in which a pilot flame is located near a main burner so that, upon the occurrence of insufficient combustion air at the main burner, the recirculation zone for combustion products which is normally located well above the burner will descend to the region occupied by the pilot flame and cause it to extinguish; a flame sensor indicating such extinction then acts to turn off the gas supply to the main burner and to the pilot burner. However, in certain straightforward applications to furnace heat exchangers, the scheme was found not to be as effective and reliable as desired.
Recent increases in the frequency of occurrence of perforations in furnace heat exchangers have been attributed to an increasing home use of products using spray-can propellants, as well as to leakage of compounds similar to spray spray propellants from compressor-type air conditioners and refrigerator freezers in the home, such materials typically comprising hallogenated hydrocarbons which tend to produce premature corroding of the metal of furnace heat exchangers.
It is therefore an object of the invention to provide new and useful apparatus for detecting inadequate levels of combustion air at a flame.
It is also an object to provide such apparatus which is reliable yet inexpensive.